This invention relates to seismic data gathering and the creation of recordings of seismic data commonly known as "seismic sections." More particularly, this invention relates to a new process for simultaneously increasing horizontal and vertical resolution while preserving high frequency response and suppressing horizontal noise in seismic sections. In addition, the present invention allows for Amplitude Variation with Offset ("AVO") analysis and removal of multiple reflections from the section, as well as providing an improved correction for static errors and conducting velocity analysis.
Early Seismic Survey Methods
For many years, the seismic industry has tried to add multiple individual signals from a particular reflection point of interest, on the theory that the summed result would have a higher signal-to-noise ratio than the individual signals, because the noise present in any particular signal is generally random, while the reflection from the point of interest is repeatable.
An example of such a process is called "vertical stacking", generally described in U.S. Pat. No. 2,018,737, issued to Owen on Oct. 29, 1935 and incorporated herein by reference. According to Owen, a plurality of relatively small charges of explosive (an example of a signal source) are successively detonated in the immediate vicinity of each other at a first location (sometimes referred to as a "shot point" or "source point"), while, at a location remote from the source point, multiple geophones (an example of a signal receiver) are used to make separate recordings of the reflections of seismic waves caused by the explosions. The records are then added. Adding the records tended to cancel random noise while increasing the repeatable signal.
Another example of using multiple source signals added together is disclosed in Peacock, et. al., "Thumping Technique Using Full Spread Of Geophones", Geophysics, Vol. 27, No. 6, Part II., pp. 952-965 (Society of Exploration Geophysicists, December 1962). Peacock describes using a thumper as a signal source to generate multiple source signals from a localized area. The area from which Peacock describes the signal's origination comprises a 10.times.10 grid of points where a heavy weight is dropped onto the earth's surface, and the resultant received signals are added together.
Peacock also disclosed a field layout of the pattern of sources and receivers commonly known as "continuous coverage", wherein, for a given minimum depth of interest, the distance between successive source points is determined to provide continuous data at a minimum depth of interest with no overlap. Generally, the distance between a first and a second source point is about equal to the offset distance between the first source point and the furthest receiver used with that source point for the shallowest geologic layer of interest. In terms normally used in the industry, the source point interval was equal to the farthest offset of the geophone spread. The continuous coverage process is also commonly known as "100%" or "single fold" profiling.
It became recognized in the art that the above-described seismic methods had inherent errors, because the signals that were added together were reflected from substantially differing locations of the geologic layer of interest. For example, Peacock describes using a 10.times.10 grid of source element locations wherein the columns of the grid were spaced 60 feet apart, giving a total area of source element locations of 560 feet by 560 feet. In practice, the large area results in no two reflections which are both added and received from the same reflection point, nor even relatively close points, although they are from the same general area. Moreover, in Peacock's method, the arrival times of signal reflections are different, due to the resultant differences in total travel path lengths. Therefore, the signals that are added by Peacock's method give both poor lateral and vertical resolution.
It will be recognized by those of skill in the art that the term "reflection point" does not refer to a mathematically precise point, but rather an area at depth on a reflecting interface from which elemental seismic reflection energy is integrated to form the total observed reflection event. In general, a reflection point as used herein refers to an area having a diameter of about 1/10th the reflector depth, with the weighting of elemental reflection energies strongest in the center of this area.
Referring still to the Peacock reference, at that time there was little or no attempt to correct for timing errors created by the differing length ray paths. An even further problem with the traditional vertical stacking was that, with continuous coverage techniques, there was little or no redundancy of measurement of a given reflection point. Further still, there were severe limitations on the process due to noise.
These and other problems are addressed, although not solved, by a process generally known as "horizontal stacking"
Later Seismic Survey Methods--Horizontal Stacking
An example of horizontal stacking is generally described in U.S. Pat. No. 2,732,906, issued to Mayne on Jan. 31, 1956 and incorporated herein by reference. Mayne teaches a radically different method of seismic data acquisition in which the signals from particular patterns of source and receiver pairs are added, wherein the subsurface reflection point for each source and receiver pair is at the same position on the reflection interface. In the case where the surface and the geologic strata are substantially flat, the required source-receiver pairs result in the source and receiver horizontal positions being located the same distance from the reflection point of interest. See FIGS. 1 and 2 of Mayne.
This horizontal stacking method, now known by those of skill in the art as the "Common Depth Point" (CDP) or, more recently, the "Common Mid-Point" (CMP) method of data acquisition (hereafter referred to collectively as "common reflection point"), was refined over the years. Eventually, the common reflection point method totally replaced the vertical stacking and continuous coverage seismic data gathering procedures exemplified by Owen and Peacock. This refinement continued with the advent of high-speed, high-memory computers, because Mayne and later horizontal stackers taught that increasing the horizontal redundancy increased the signal-to-noise ratio. More and more receiver groups were used, and source points were moved closer and closer, such that there were many source points within the total spread of geophone groups, giving more and more traces from differing pairs of source points and receiver points that could be added together. As used herein, the term "traces" refers to the graphical representations of signals received by receivers, as well as their electronic equivalents, as represented in a computer.
Attributes Of Horizontal Stacking
AVO Analysis
One of the benefits of horizontal stacking, compared with the earlier prior art, was the ability to perform Amplitude Variation with Offset ("AVO") analysis.
It will be recognized by those of skill in the art that before normal moveout correction of the traces gathered together in the common reflection point method, some traces are from source-receiver pairs that are further apart from each other than are the source-receiver pairs for other traces. As said by those of skill in the art, some traces have further offsets than others. If the traces are graphed on a grid having a horizontal scale in seconds of time between the source event and the reception of the signal that the trace represents and a vertical scale in feet of offset, the reflection signals for any particular reflection point will be on an approximate hyperbolic curve 1A1 (FIG. 1A), increasing in time as offset increases. This is called the "moveout curve". The common reflection point set of traces is typically called a "gather". Further, the amplitude of the reflection may change as offset increases. After normal moveout, the traces are added, and the result is graphed in the seismic section as a single trace, along with all other moveout-corrected traces from all other common reflection points. In the seismic section, the positive excursion-of the traces are darkened, and the negative values are traced uncolored. The result (seen in FIG. 1B) shows correlative sequences of reflections from subsurface layers of rock interfaces plotted against time of arrival as a vertical axis and horizontal distance along the surface of the earth (or some selected datum).
Interpreters look for darkened areas under which they expect to find oil or gas. However, the darkened areas over which there may be oil or gas can look (in the common reflection point method) just like the darkened areas over which brine water will be found. Fortunately, however, it has been found that the amplitude of the reflection for any single trace from a single source-receiver pair will change with the offset distance between the source and receiver, and it will change differently for brine water than it will for a hydrocarbon. Therefore, interpreters will look to the individual traces in the gather, to analyze how the amplitude of the reflection changes with offset.
Such AVO analysis was not available with single coverage methods, because there were not multiple traces of reflections from the same reflection point.
Suppression Of Multiple Reflections
Another problem with single coverage methods was that of multiple reflections.
As will be understood by those of skill in the art, signals reflecting from one reflection point will travel back to the surface, or a higher transition layer, and reflect down, again. When they reach the original level from which they first reflected, they will again reflect upward, at which point they may be received at the surface. However, due to the extra travel path length for multiple reflections, the multiple reflection signal arrives at the receiver at a later time than its primary arrival time, which may correspond to other primary reflection signals from reflection points below the multiple reflection signal's original reflection level. Thus, unless multiple reflections are suppressed, the eventual seismic section will show false structure at depths where there may be no structure, or at least degrade the quality of the primary reflection data from deep levels with which such multiple reflections interfere. With single coverage methods, there was no capability for multiple reflection suppression, and the interpreter had to determine from the characteristics of the data what was a multiple and what was a primary reflection event.
In the common reflection point method, it was found that the moveout curve for multiple reflections was different from the moveout curve of the primary reflections arriving at about the same time as the multiple. Accordingly, if the difference in offset between the shortest offset trace and the longest offset trace was sufficient, then the synchronization of the traces in the gather according to the primary reflection hyperbola would cause the multiple reflections to appear as under corrected primary events. Accordingly, when the normal moveout-corrected traces in a gather were added to form a single trace element of the seismic section, the multiple reflections, not being equally well synchronized, were suppressed relative to the synchronized primary reflector.
Again, without a multiplicity of traces from source-receiver pairs having a significant difference in offset, multiples could not be suppressed. Modern-day common reflection point methods use many channels (as many as 960 or more) and long offsets (typically as much as 20,000 feet), thus allowing for both AVO and multiple reflection suppression. However, modern-day methods introduce other inaccuracies.
Inaccuracies In The Common Reflection Point Method
Some of the assumptions used in the signal processing steps necessary for use of the prior art common reflection point method have been found to be inaccurate. For example, the signal path for source-receiver pairs used in the common reflection point methods do not all travel at the same velocity. If the signals were moving through homogeneous material, path length differences would cause the moveout curve to be exactly hyperbolic, and the timing corrections using the hyperbolic curve of the primary reflections in the gather (typically called "normal moveout" or "NMO") would be accurate. However, the real world is not homogeneous. Therefore, in many situations, the velocity of propagation is different for the various paths of travel and for various parts of each path. These errors are commonly termed "non-hyperbolic moveout errors" and "static errors".
To address this problem in the prior art, the common reflection point method applies time correction to the data, based on simple (and known to be inaccurate) models. NMO is one example of a synchronization process, where differences in common reflection point arrival times are computed based on a simple model. In that model, one homogeneous layer from the surface to the reflector and a common average velocity in this layer for all ray paths with the common reflection point are presumed. The time of signal reception for any source-receiver pair with a given offset distance is assumed to be defined by a second order equation which expresses the observed arrival time as a function of a common source-receiver position at the midpoint of the pair, the offset distance between the pair, and the average velocity of propagation over the entire travel path. This velocity of propagation is assumed to be the same for all travel paths associated with data in a common reflection point "gather". The NMO process also assumes that the velocity of propagation for any given event is a constant value equal to the root-mean-square average of all local layer velocities along the path of propagation for all offset observations exhibited in the gather. These assumptions are known to be in error.
As another example, signals traveling through the materials within about 500 feet of the surface, either down-going from the source or up-going to the receiver, are subjected to considerable local travel-time differences, because of the heterogeneity of shallow, unconsolidated or altered materials. These timing errors are called "sub-weathering static errors", and another synchronization process is applied in the prior art common reflection point method to try to correct for such errors before adding signals in the gathers. According to the sub-weathering synchronization process, a simple model of vertically-traveling energy near the surface at both source and receiver locations is used. These estimates always have some residual error, partly because of the failure of this simple model to hold, and because of additional noise in the measurements used to make the model. The identity of locations along the seismic profile where these residual errors occur is obscured when the data is added, resulting not only in degradation of the data quality, but also in eliminating any ability to clearly observe and take such errors into account when the data is interpreted.
One very common source of non-random, but indeterminable, error in the prior art common reflection point methods exists due to localized geologic anomalies in the subsurface. For example, FIG. 1C shows a typical set of ray paths 10a-10t for a common reflection point process for a particular reflection point 12 at sub-surface transition layer 13. The assumptions used to synchronize the ray paths 10a-10t are not able to correct for the velocity changes caused by localized anomalous geologic formations 14a-14c. Those velocity changes will occur for some seismic signals, but not all, giving a smeared result after addition. Further, the types of anomalies 14a-14c (or even their existence) are difficult to detect, because the anomalies 14a-14c do not intersect all, or even most, of the ray paths 10a-10t in the prior art common reflection point method. Thus, there is a need to have some method to effectively utilize redundancy without introduction of error caused by anomalous geologic formations in the various propagation paths of data being added.
Addition of signals after imperfect synchronization results in the loss of higher frequencies, the loss of resolving power, and (possibly more importantly) the loss of the seismic interpreter's ability to detect that error exists. Without detection of the error, there is no ability to pinpoint its cause or take the cause into account during interpretation. Therefore, there is inherent error introduced by the prior art common reflection point method. Such error cannot be accommodated by existing practice.
This class of non-random but unmeasurable errors in the common reflection point stacking methods give rise to attempts to reduce such errors by the use of complicated processes such as multi-channel stacking filters, discussed in Galbraith, J. N., "Characteristics of Optimum Multichannel Stacking Filters", Geophysics, Vol. 33, No. 1, pp. 36-48 (February 1968), incorporated herein by reference. Such extra complexity is not desirable, and such filtering can also remove information that is desired.
Horizontal Noise
Another major problem in all seismic exploration is that of a non-random but complex phenomenon called "horizontal noise" (a.k.a. source-generated noise). Horizontal noise comprises seismic waves radiating radially away from the source points traveling through the near-surface layers of the earth, and are generated by the source. Such waves travel at a speed slower than the reflected seismic waves of interest. Therefore, for any given depth-point of interest, there is a distance from the source along the surface at which the reflected seismic wave will arrive substantially at the same time as the horizontal noise. Further, horizontal noise has many components, each of which travels at a slightly different speed, causing the composite of all such noise modes to appear longer in time duration and to be worse at the greatest source-receiver distances. To a significant degree, some form of horizontal noise is coincident with nearly all reflections at all offsets on all records.
In the prior art common reflection point methods described above, receiver and source arrays have been proposed to reduce the effect of horizontal noise, as is known to those of skill in the art and described in Anstey, N. A., "What Ever Happened To Ground Roll?", Geophysics: The Leading Edge of Exploration, pp. 40-45, March 1986, incorporated herein by reference. Anstey notes that, in the field, in the common reflection point method, the accepted practice is to use a geophone group length (group length being defined as the distance over which elements of the geophone group are placed) designed to suppress horizontal noise. The group length is, in practice, about the same as the geophone group interval (group interval being defined as the distance between the center of two geophone groups). Anstey notes that for the prior art common reflection point practice, the receiver array must have a length equal to twice the source interval and/or vice versa. He also notes that the source interval (the distance between the center of two source points) should be 10 half of the group interval for end-on spreads and equal to the group interval for split spreads. Anstey further notes that the goal in the prior art common reflection point gather method for suppressing horizontal noise is to have a continuous, uniform succession of sampling points across the common reflection point gather (hereinafter, sometimes referred to as the Anstey "stack array"). However, in practice, the Anstey stack array has been rejected, because it is too expensive.
The excessive expense exists because of competing factors in the suppression of horizontal noise. Those competing factors include lateral resolution and high frequency response. For example, a client for seismic data will specify a maximum frequency of interest (hereinafter designated as "f.sub.max "), which must not be degraded by more than a particular percentage, as understood by those of skill in the art. As the geophone group length goes up to suppress horizontal noise, the high frequency response goes down. Also, to increase the lateral resolution of the data, common wisdom suggests that receiver group intervals should be short. With short group lengths for preserving high frequencies, there is the necessity for even shorter group intervals.
Shooting or vibrating sources every group interval (as is the requirement if the group length is twice the group interval), as suggested by Anstey, requires a source point spacing equal to the group interval. The number of source points required per unit of length of the seismic profile becomes excessively large, and the expense of acquisition becomes prohibitive. Therefore, common practice is to place a source point every second to fourth group interval for a split spread of (typically) 120 or more groups having group lengths of 80 to 110 feet. Further, keeping the same example distances, Anstey requires receiver group intervals of 160 to 220 feet for sources placed at every other group interval, or 320 to 440 feet for sources placed every fourth group interval. Such a requirement severely damages high frequencies in the recorded data.
It can be seen from the above discussion that with traditional vertical stacking methods, and the horizontal stacking methods that replaced vertical stacking, seismic data gathering techniques are unavailable that are capable of suppressing horizontal noise while still providing high lateral resolution and high frequency response. As will be more fully described below, the present invention provides a solution to this long-standing trade off. Further, the methods that suppress multiple reflections and allow for AVO analysis actually introduce error due to their known-to-be incorrect assumptions in synchronization. The present invention addresses these problems and tradeoffs.